1. Field of the Invention
This invention relates generally to the field of computer graphics, and more specifically to an apparatus and method for evaluating and rendering parametric surfaces.
2. Background Art
The evolution of computer technology has resulted in the creation of a sophisticated technical art devoted to the representation of graphical information generated by computers. This art is referred to as computer graphics. In recent years, the use of 3-dimensional computer graphics in scientific and engineering applications has increased, along with the demand for realistic images.
Besides lines, markers and polygons, computer graphics displays today support more general geometric primitives, such as parametric surfaces. However, the methods used, at present, for evaluating and rendering such surfaces in computer graphics display systems are often inefficient and require complicated and extensive external control logic.
It is known that parametric surfaces can be represented in b-spline form and others have attempted, in the past, to evaluate and render surfaces therefrom. For example, a conventional CAD/CAM application uses a graphics workstation by sending down polygons which make up objects stored in the host computer data base, rather than the b-spline surface form data stored in the data base. According to this procedure, the host computer decomposes the spline surface into flat polygons, and then sends the polygons to the graphics workstation for conventional processing and subsequent display. This procedure is inefficient and fails to take advantage of the many attractive attributes of the b-splines.
Commonly assigned U.S. patent application Ser. No. 07/115,465, filed Oct. 30, 1987 now U.S. Pat. No. 4,912,659, describes a parallel pipeline surface processing system which decomposes non-uniform rational b-spline (NURBS) surfaces into rational Bezier patches for subsequent processing. This processing by patch approach requires computational resources for the decomposition step and risks the appearance of artifacts (e.g. pin holes or rips) in the surface rendered on the screen.
None of the known prior art approaches fully capitalize on NURBS data in evaluating and rendering parametric surfaces. Accordingly, a need exists for a method and apparatus for evaluating and rendering NURBS data representative of a parametric surface, in an efficient, accurate and rapid fashion. The proposed PHIGS (Programmer's Hierarchical Interactive Graphics Standard) PLUS standard for interactive 3-dimensional graphics, supports the definition of surfaces in terms of NURBS and, thus, reinforces this need.